1. Field of the Invention
The present invention relates to an optical transceiver usable for both of optical fiber transmission and free space transmission as well as to an optical communications network where both of optical fiber transmission and free space transmission can be used.
2. Description of the Related Art
There have been made many attempts to use optical communication or optical transmission for short-distance signal transmission. Among those attempts are an optical LAN (local area network) in which optical communication is applied to a LAN, an optical bus in which optical transmission is applied to an external bus for connection between a computer and peripheral devices, and a wireless communications system using optical transmission.
FIG. 38A shows an example of a wireless communications system using optical transmission, which is an optical transmission system using infrared light that is propagated through a free space. Optical communication is performed between terminal stations 110 by means of free space light. Examples of this type of system are an IrDA (infrared data association) system and an ASK (amplitude shift keying) system (see Nikkei Electronics 1995.2.13 (No. 628), pp. 101-110). The IrDA system is a point-to-point optical communications system, and employs RZ (return to zero) modulation and dipulse coding. The RZ modulation means a scheme in which light is not output during absence of a transmission request. The dipulse coding means a coding scheme in which pulse widths or positions are defined for "1" and "0."
FIG. 38B shows a point-to-point optical transmission system using an optical fiber. Optical communication is performed between terminal stations 112 by means of light that is propagated by an optical fiber 113. In this connection, a technique called "fiber channel" is known (see Nikkei Electronics 1994.1.17 (No. 599), pp. 127-137), which can be used in a LAN or an external bus. The fiber channel technique employs NRZ (non-return to zero) modulation and block coding. The NRZ modulation means a scheme in which light is output even during absence of a transmission request. The block coding means a coding scheme in which 4-bit data are allocated to 5-bit patterns such that the possibility of continuous occurrence of 1's or 0's is minimized.
FIG. 38C is an optical communications system using a star coupler, in which optical communication is performed between terminal stations 114 via a broadcasting bus system that is formed by using a star coupler 115. In contrast to the case of FIG. 38B, this optical communications system enables a one-vs.-many communication. Related techniques are described in Japanese Unexamined Patent Publication Nos. Sho. 58-90843, Hei. 3-296,332, Hei. 4-372909, and Hei. 5-3457, U.S. Pat. No. 5,282,257, and a paper by Takeshi Ohta, "Four-port Multimode Interconnectable Star Coupler," Electronics Letters, Vol. 29, No. 10, pp. 919-920, 1993. Also known is an optical communications system using a broadcasting bus system in which wavelength multiplexing is effected. Related techniques are described in Japanese Unexamined Patent Publication Nos. Hei. 2-98253, Hei. 2-162939, Hei. 3-102932, Hei. 3-270432, and Hei. 5-14385, and U.S. Pat. No. 5,144,466.
FIG. 39 show details of an example of the optical communications network of FIG. 38C using a star coupler. In FIG. 39, reference numerals 126a and 126b denote optical fibers; 127, stations; 125, a mixing-rod-type star coupler; and 124, terminals. Electrical signals that are output from the respective stations 127 are converted by light-emitting elements 122 into optical signals, which are supplied to the star coupler 125 via the optical fibers 126a. After being mixed by the star coupler 125, the optical signals are distributed to photodetecting elements 126b via the optical fibers 126b. The optical signals are converted by the photodetecting elements 123 to electrical signals, which are input to the respective stations 127. Constructed in this manner, this network has a feature that a signal that is output from one station is transmitted to the other stations, i.e., a broadcasting feature.
However, the conventional star coupler 125 (not interconnectable) of the above network has a feature that a signal transmitted from one station is also distributed to its own reception port. This feature makes collision detection difficult for the following reason. Since in general the passive star coupler 125 is insufficient in the uniformity of the distribution ratio, it is difficult to use, as a collision detection method, a level difference method which is used in, for instance, an Ethernet with a coaxial cable.
To solve this problem, a collision detection method based on a code rule violation (CRV) technique has been proposed for use in the network as shown in FIG. 39 (see K. Oguchi and Y. Hakamada: "New Collision Detection Technique and its Performance," Electronics Letters, Vol. 20, No. 25/26, pp. 1,062-1,063, 1984).
The code rule violation method utilizes the fact that in Manchester coding used in an Ethernet, 1-bit data is represented by a 2-bit code, that is, the Manchester coding is redundant.
FIG. 38D shows a network in which terminal stations 116 are connected to each other in a daisy-chain-like manner via optical fibers 113. Related techniques are described in Japanese Unexamined Patent Publication No. Hei. 2-140025, and U.S. Pat. Nos. 4,948,218 and 4,747,651.
Although the above-described optical communications systems and transmission systems are similar to each other, they have almost no interconnectability or compatibility at present. For examples because of the use of NRZ modulation, the terminal station 112 of the point-to-point optical transmission system of FIG. 28B using an optical fiber cannot be used in the free space transmission system of FIG. 38A or the network of FIG. 38C using a star coupler.
By the way, in wireless communications systems using optical transmission, there is a concern that transmission light may enter the eyes of a-person and impair his health.
Furthers there is a technical trend that a graded-index (GI) plastic optical fiber having a large core diameter (0.5 mm) is attracting much attention (see T. Ishigure et al.: "Graded-index Polymer Optical Fiber for High-speed Data Communication," Applied Optics, Vol. 33, No. 19, pp. 4261-4266, 1994). It is expected that the graded index profile will enable high-speed transmission, and that the thick core will greatly reduce the cost of an optical connector.